For more than three decades, fly ash has been one of the world's most important supplementary cementitious materials (SCMs), enabling the production of low-carbon concrete by replacing a portion of Portland cement. But the global transition away from coal-fired power plants is creating an unexpected challenge: the supply of high-quality fly ash is rapidly declining.
This raises an important question for researchers and the construction industry alike:
What will replace fly ash in the next generation of sustainable cement?
A recent study by Shahin et al. (2026), recently published in Discover Materials, investigates whether mechanically activated Mine Waste Rock (MWR), an abundant but largely discarded mining by-product, can become a practical fly ash alternative for cement production.
Why Mine Waste Rock Matters
Mining operations generate enormous quantities of waste rock every year. Worldwide stockpiles are estimated to exceed 200 billion tonnes, occupying valuable land while creating long-term environmental challenges.
Unlike conventional SCMs, however, mine waste rock is not naturally reactive.
It is coarse, highly crystalline, and behaves largely as an inert aggregate.
The challenge therefore is not simply using the material, it is engineering its reactivity.
Mechanical Activation Unlocks Hidden Reactivity
Our research demonstrates that high-energy ball milling can fundamentally change the behaviour of mine waste rock.
By reducing the median particle size to approximately 40 μm, mechanical activation increases surface area, introduces lattice defects, and enables the siliceous phases to participate in cement hydration.
Rather than acting solely as an inert filler, the activated material contributes to hydration and microstructural development.
This finding suggests that mechanically activated Ground Recycled Mine Waste Rock Blended Cement (GRMBC) may represent a new class of sustainable supplementary cementitious material.
Key Findings
The study evaluated GRMBC at replacement levels between 10% and 50%.
The results showed:
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Comparable compressive strength to ordinary Portland cement at 10–20% replacement
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Balanced structural performance and resource efficiency at 40% replacement
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20–30% lower CO₂ emissions per MPa of strength
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20–30% lower material cost per MPa
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Evidence from isothermal calorimetry, FTIR, TGA, SEM and porosity analysis indicating continued hydration, secondary C–S–H formation and matrix densification.
Rather than behaving as a passive filler, the activated mine waste exhibited measurable participation in hydration, particularly at later ages.
Why This Research Matters
The future of low-carbon cement cannot depend indefinitely on industrial by-products that are themselves becoming scarce.
As the availability of fly ash and blast furnace slag continues to decline, construction materials researchers are increasingly investigating locally available mineral resources and industrial wastes.
Mine waste rock is particularly attractive because of its enormous availability and its potential to simultaneously address two environmental challenges:
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reducing cement-related carbon emissions
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diverting mining waste from long-term stockpiles
If successfully implemented at scale, this approach could support circular economy strategies while reducing dependence on virgin limestone resources.
A New Direction for Sustainable Cement
The future of sustainable concrete will depend on expanding the portfolio of high-performance supplementary cementitious materials.
This research demonstrates that mechanically activated Mine Waste Rock can help meet that challenge, transforming an abundant mining by-product into a practical, low-carbon binder capable of supporting the next generation of resilient infrastructure and circular construction.
Paper
Published in Discover Materials (Springer Nature) - https://doi.org/10.1007/s43939-026-00745-w